Green’s Dictionary of Slang

a formal tailcoat; thus white tie and tails.

1864		Town Crier 23 Mar. in Rock Is Argus (15 Nov. 1943) Your father said he forgot to tell you to get some tails (dress suit).
1888		St Nicholas xiv 406: Once a boy has reached the modern remove [Harrow], he puts on his tails or tail-coat [F&H].
1938		in W.C. Fields By Himself (1974) 262: whipsnade: What tails? phineas: Your evening clothes.
1975		D. Nobbs Fall and Rise of Reginald Perrin (1976) 100: Good lord! You’re in tails.
1979		B. Gutcheon New Girls (1982) 272: George’d be perfect. He owns a set of tails.
2000		Indep. on Sun. 23 Jan. 10: The first prize was a day at Royal Ascot, top hat and tails provided.